Traumatic brain injury (TBI) is a degenerative process, with an initial primary injury which causes immediate mechanical cell death. This injury also induces biochemical and cellular changes that contribute to continuing neuronal damage and death over time. This continuing damage is known as secondary injury, and multiple apoptotic and inflammatory pathways are activated as part of this process. One of the neurotoxic elements produced following TBI is the Alzheimer's disease-related protein A[unreadable]. A[unreadable] deposits, similar to those in Alzheimer's disease, are seen within 24 hours after exposure to TBI. A[unreadable] is produced following sequential cleavage of the amyloid precursor protein (APP) by [unreadable]- and ?-secretase. We have recently reported that A[unreadable] and the APP secretases are elevated in non-transgenic mice following TBI, with protein levels peaking at 3 days post-trauma. We found that immediate treatment with a ?- secretase inhibitor (DAPT) can completely block the learning deficits following TBI, and reduce brain lesion volume by 70%. Thus, we conclude that ?- secretase is a promising target for treatment of TBI. In order to fully exploit this new target, a key set of experiments have been designed. Firstly, the therapeutic window for APP secretase inhibition will be calculated. By narrowing the treatment window (both the start and end points of treatment), we can determine the time at which APP secretases are initiating secondary injury, and determine how long treatment should be maintained. This data will help us identify where in the sequence of secondary injury that APP secretases are important, and help to establish a therapeutic strategy for this class of inhibitors. Secondly, it is unclear from our data what the downstream target of APP secretase inhibitors are. Aim 2 of this application examines APP and A[unreadable] as primary downstream targets of ?-secretase following trauma. While between them [unreadable]- and ?-secretase have multiple downstream targets, there are a limited set of proteins that are cleaved by both [unreadable]- and ?-secretase. Given the excess of data suggesting that A[unreadable] can impair blood flow, induce inflammation and cause apoptosis - all hallmarks of secondary injury - APP/A[unreadable] is the most apparent of these targets. The specific aims are designed to enhance our understanding of ?- secretase inhibitors as a treatment for TBI, and to determine if the continuing cell death following TBI is mediated through APP processing. PUBLIC HEALTH RELEVANCE: This application aims to establish the role of the Alzheimer's disease peptide A[unreadable] after traumatic brain injury (TBI). A[unreadable] levels rapidly increase after TBI in the days following injury. We have previously shown that blocking [unreadable]- and ?- secretase activity after TBI can reduce hippocampal cell death and prevent learning impairments after trauma. Both [unreadable]- and ?- secretase inhibitors are currently in clinical trials for Alzheimer's disease, making these drugs a novel therapeutic strategy for TBI. This application will enhance our understanding of APP-secretase inhibitors as a treatment for TBI, and determine if the continuing cell death following TBI is mediated through cleavage of APP or production of the A[unreadable] peptide.